Systems for Wide-Band Transmission Over
Coaxial Lines
By L. ESPENSCHIED and M. E. STRIEBY
In this paper systems are described whereby frequency band widths of
the order of 1000 kc. or more may be transmitted for long distances over
coaxial lines and utilized for purposes of multiplex telephony or television.
A coaxial line is a metal tube surrounding a central conductor and separated
from it by insulating supports.
FT appears from recent development work that under some condi-
■*• tions it will be economically advantageous to make use of consider-
ably wider frequency ranges for telephone and telegraph transmission
than are now in use l > 8 or than are covered in the recent paper on carrier
in cable. 3 Furthermore, the possibilities of television have come into
active consideration and it is realized that a band of the order of one
million cycles or more in width would be essential for television of
reasonably high definition if that art were to come into practical
use. 4,6
This paper describes certain apparatus and structures which have
been developed to employ such wide frequency ranges. The future
commercial application of these systems will depend upon a great
many factors, including the demand for additional large groups of
communication facilities or of facilities for television. Their prac-
tical introduction is, therefore, not immediately contemplated and, in
any event, will necessarily be a very gradual process.
Types of High-Frequency Circuits
The existing types of wire circuits can be worked to frequencies of
tens of thousands of cycles, as is evidenced by the widespread applica-
tion of carrier systems to the open-wire telephone plant and by the
development of carrier systems for telephone cable circuits. 2,3 Fur-
ther development may lead to the operation of still higher frequencies
over the existing types of plant. However, for protection against
external interference these circuits rely upon balance, and as the
frequency band is widened, it becomes more and more difficult to
maintain a sufficiently high degree of balance. The balance require-
ments may be made less severe by using an individual shield around
1 For references, see end of paper.
654
* Published in Electrical Engineering, October, 1934. Scheduled for presentation
at Winter Convention of A. I. E. E., New York, N. Y., January, 1935.
WIDE-BAND TRANSMISSION OVER COAXIAL LINES 655
each circuit, and with sufficient shielding balance may be entirely
dispensed with.
A form of circuit which differs from existing types in that it is un-
balanced (one of the conductors being grounded), is the coaxial or
concentric circuit. This consists essentially of an outer conducting
tube which envelops a centrally-disposed conductor. The high-
frequency transmission circuit is formed between the inner surface of
the outer conductor and the outer surface of the inner conductor.
Unduly large losses at the higher frequencies are prevented by the
nature of the construction, the inner conductor being so supported
within the tube that the intervening dielectric is largely gaseous, the
separation between the conductors being substantial, and the outer
conductor presenting a relatively large surface. By virtue of skin
effect, the outer tube serves both as a conductor and a shield, the
desired currents concentrating on its inner surface and the undesired
interfering currents on the outer surface. Thus, the same skin effect
which increases the losses within the conductors provides the shield-
ing which protects the transmission path from outside influences, this
protection being more effective the higher the frequency.
The system which this paper outlines has been based primarily upon
the use of the coaxial line. The repeater and terminal apparatus
described, however, are generally applicable to any type of line, either
balanced or unbalanced, which is capable of transmitting the frequency
range desired.
The Coaxial System
A general picture of the type of wide band transmission system which
is to be discussed is briefly as follows: A coaxial line about 1/2 inch in
outside diameter is used to transmit a frequency band of about
1,000,000 cycles, with repeaters capable of handling the entire band
placed at intervals of about 10 miles. Terminal apparatus may be
provided which will enable this band either to be subdivided into more
than 200 telephone circuits or to be used en bloc for television.
Such a wide-band system is illustrated in Fig. 1. It is shown to
comprise several portions, namely, the line sections, the repeaters, and
the terminal apparatus, the latter being indicated in this case as for
multiplex telephony. Two-way operation is secured by using two
lines, one for either direction. It would be possible, however, to
divide the frequency band and use different parts for transmission in
opposite directions.
A form of flexible line which has been found convenient in the ex-
perimental work is illustrated in Fig. 2 and will be described more fully
656
BELL SYSTEM TECHNICAL JOURNAL
TERMINAL
MULTIPLEXING
APPARATUS
TERMINAL
MULTIPLEXING
APPARATUS
LINES AND REPEATERS
==l,l
U="
Fig. 1 — Diagram of coaxial system.
subsequently. Such a coaxial line can be constructed to have the
same degree of mechanical flexibility as the familiar telephone cable.
While this line has a relatively high loss at high frequencies, the trans-
mission path is particularly well adapted to the frequent application
of repeaters, since the shielding permits the transmission currents to
fall to low power levels at the high frequencies.
Of no little importance also is the fact that the attenuation-fre-
quency characteristic is smooth throughout the entire band and obeys
a simple law of change with temperature. (This is due to the fact that
the dielectric is largely gaseous and that insulation material of good
dielectric properties is employed.) This smooth relation is extremely
Fig. 2 — Small flexible coaxial structure.
helpful in the provision of means in the repeaters for automatically
compensating for the variations which occur in the line attenuation
with changes of temperature. This type of system is featured by
large transmission losses which are offset by large amplification, and it
is necessary that the two effects match each other accurately at all
times throughout the frequency range.
It will be evident that the repeater is of outstanding importance in
this type of system, for it must not only transmit the wide band of
frequencies with a transmission characteristic inverse to that of the
line, with automatic regulation to care for temperature changes, but
must also have sufficient freedom from inter-modulation effects to
permit the use of large numbers of repeaters in tandem without objec-
WIDE-BAND TRANSMISSION OVER COAXIAL LINES 657
tionable interference. Fortunately, recent advances in repeater tech-
nique have made this result possible, as will be appreciated from the
subsequent description.
An interesting characteristic of this type of system is the way in
which the width of the transmitted band is controlled by the repeater
spacing and line size, as follows:
1. For a given size of conductor and given length of line, the band
width increases nearly as the square of the number of the re-
peater points. Thus, for a coaxial circuit with about .3-inch
inner diameter of outer conductor, a 20-mile repeater spacing
will enable a band up to about 250,000 cycles to be transmitted,
a 10-mile spacing will increase the band to about 1,000,000
cycles, and a 5-mile spacing to about 4,000,000 cycles.
2. For a given repeater spacing, the band width increases approxi-
mately as the square of the conductor diameter. Thus, whereas
a tube of .3-inch inner diameter will transmit a band of about
1,000,000 cycles, .6-inch diameter will transmit about 4,000,000
cycles, while a diameter corresponding to a full-sized telephone
cable might transmit something of the order of 50,000,000
cycles, depending upon the dielectric employed and upon the
ability to provide suitable repeaters.
Earlier Work
It may be of interest to note that as a structure, the coaxial form of
line is old— in fact, classical. During the latter half of the last century
it was the object of theoretical study, in respect to skin effect and other
problems, by some of the most prominent mathematical physicists of
the time. Reference to some of this work is made in a paper by
Schelkunoff, dealing with the theory of the coaxial circuit. 6
On the practical side, it is found on looking back over the art that the
coaxial form of line structure has been used in two rather widely differ-
ent applications: first, as a long line for the transmission of low fre-
quencies, examples of which are usage for submarine cables, 7, 8 and for
power distribution purposes, and second as a short-distance, high-
frequency line serving as an antenna lead-in. 9, 10
The coaxial conductor system herein described may be regarded as
an extension of these earlier applications to the long-distance trans-
mission of a very wide range of frequencies suitable for multiplex
telephony or television. 11 Although dealing with radio frequencies,
this system represents an extreme departure from radio systems in that
a relatively broad band of waves is transmitted, this band being con-
658 BELL SYSTEM TECHNICAL JOURNAL
fined to a small physical channel which is shielded from outside dis-
turbances. The system, in effect, comprehends a frequency spectrum
of its own and shuts it off from its surroundings so that it may be used
again and again in different systems without interference.
This new type of facility has not yet been commercially applied. It
is, in fact, still in the development stage. Sufficient progress has
already been made, however, to give reasonable assurance of a satis-
factory solution of the technical problems involved. This progress
is outlined below under three general headings: (1) the coaxial line and
its transmission properties, (2) the wide band repeaters, and (3) the
terminal apparatus.
The Coaxial Line
An Experimental Verification
One of the first steps taken in the present development was in the
nature of an experimental check of the coaxial conductor line, de-
signed primarily to determine whether the desirable transmission prop-
erties which had been disclosed by a theoretical study could be fully
realized under practical conditions. For this purpose a length of
coaxial structure capable of accurate computation was installed near
Phoenixville, Pa. Figure 3 shows a sketch of the structure used and
gives its dimensions. It comprised a copper tube of 2.5 inches outside
diameter, within which was mounted a smaller tube which, in turn,
contained a small copper wire. Two coaxial circuits of different sizes
were thus made available, one between the outer and the inner tubes,
and the other between the inner tube and the central wire. The
instalation comprised two 2600-foot lengths of this structure.
The diameters of these coaxial conductors were so chosen as to ob-
tain for each of the two transmission paths a diameter ratio which
approximates the optimum value, as discussed later. The conductors
were separated by small insulators of isolantite. The rigid construc-
tion and the substantial clearances between conductors made it pos-
sible to space the insulators at fairly wide intervals, so that the dielec-
tric between conductors was almost entirely air. The outer conductor
was made gas-tight, and the structure was dried out by circulating
dry nitrogen gas through it. The two triple conductor lines were
suspended on wooden fixtures and the ends brought into a test house,
as shown in Fig. 4.
The attenuation was measured by different methods over the fre-
quency range from about 100 kilocycles to 10,000 kilocycles. In-
vestigation showed that the departures from ideal construction occa-
sioned by the joints, the lack of perfect concentricity, etc., had remark-
WIDE-BAND TRANSMISSION OVER COAXIAL LINES
659
ably little effect on the attenuation. In order to study the effect of
eccentricity upon the attenuation, tests were made in which this effect
was much exaggerated, and the results substantiated theoretical pre-
dictions. The impedance of the circuits was measured over the same
range as the attenuation. A few measurements on a short length
were made at frequencies as high as 20,000 kilocycles.
SPACING OF INSULATORS
LARGE SIZE : 4 FEET ON STRAIGHTAWAY
2 FEET ON CURVES
SMALL SIZE : I FOOT ON STRAIGHTAWAY
6 INCHES ON CURVES
Fig. 3 — Structure used in Phoenixville installation.
Measurements were secured of the shielding effect of the outer con-
ductor of the coaxial circuit up to frequencies in the order of 100 to
150 kilocycles, the results agreeing closely with the theoretical values.
Above these frequencies, even with interfering sources much more
powerful than would be encountered in practice, the induced currents
dropped below the level of the noise due to thermal agitation of elec-
tricity in the conductors (resistance noise) and could not be measured.
The preliminary tests at Phoenixville, therefore, demonstrated that
660
BELL SYSTEM TECHNICAL JOURNAL
Fig. 4 — Phoenixville installation showing conductors entering test house.
a practical coaxial circuit, with its inevitable mechanical departures
from the ideal, showed transmission properties substantially in agree-
ment with the theoretical predictions.
Small Flexible Structures
Development work on wide-band amplifiers, as discussed later,
indicated the practicability of employing repeaters at fairly close in-
tervals. This pointed toward the desirability of using sizes of coaxial
circuit somewhat smaller than the smaller of those used in the pre-
liminary experiments, and having correspondingly greater attenua-
tion. Furthermore, it was desired to secure flexible structures which
could be handled on reels after the fashion of ordinary cable. Ac-
cordingly, several types of flexible construction, ranging in outer
diameter from about .3 inch to .6 inch, have been experimented with.
Structures were desired which would be mechanically and electrically
satisfactory, and which could be manufactured economically, prefer-
ably with a continuous process of fabrication.
One type of small flexible structure which has been developed is
shown in Fig. 2. The outer conductor is formed of overlapping copper
strips held in place with a binding of iron or brass tape. The insula-
tion consists of a cotton string wound spirally around the inner con-
ductor, which is a solid copper wire. This structure has been made in
several sizes of the order of 1/2 inch diameter or less. When it is to be
used as an individual cable, the outer conductor is surrounded by a
WIDE-BAND TRANSMISSION OVER COAXIAL LINES 661
lead sheath, as shown, to prevent the entrance of moisture. One or
more of the copper tape structures without individual lead sheath may
be placed with balanced pairs inside a common cable sheath.
Another flexible structure is shown in Fig. 5. The outer conductor
in this case is a lead sheath which directly surrounds the inner conduc-
tor with its insulation. Since lead is a poorer conductor than copper,
it is necessary to use a somewhat larger diameter with this construction
in order to obtain the same transmission efficiency. Lead is also in-
ferior to copper in its shielding properties and to obtain the same de-
gree of shielding the lead tube of Fig. 5 must be made correspondingly
thicker than is necessary for a copper tube.
The insulation used in the structure shown in Fig. 5 consists of hard
RUBBER WASHER >NNB* CONDUCTOR
(copper;
Fig. 5 — Coaxial structure with rubber disc insulators.
rubber discs spaced at intervals along the inner wire. Cotton string
or rubber disc insulation may be used with either form of outer tube.
The hard rubber gives somewhat lower attenuation, particularly at the
higher frequencies.
Another simple form of structure employs commercial copper tubing
into which the inner wire with its insulation is pulled. Although this
form does not lend itself readily to a continuous manufacturing process,
it may be advantageous in some cases.
Transmission Characteristics
Attenuation
At high frequencies the attenuation of the coaxial circuit is given
closely by the well-known formula:
R lC , G [L
a = -=^ ~
2\Z" r 2\C
(1)
where R, L, C and G are the four so-called " primary constants" of the
line, namely, the resistance, inductance, capacitance and conductance
662
BELL SYSTEM TECHNICAL JOURNAL
per unit of length. The first term of (1) represents the losses in the
conductors, while the second term represents those in the dielectric.
When the dielectric losses are small, the attenuation of a coaxial
circuit increases, due to skin effect in the conductors, about in accord-
ance with the square root of the frequency. With a fixed diameter
ratio, the attenuation varies inversely with the diameter of the circuit.
By combining these relations there are obtained the laws of variation
of band width in accordance with the repeater spacing and the size of
circuit, as stated previously.
The attenuation-frequency characteristic of the flexible structure
illustrated in Fig. 2, with about .3 inch diameter, is given in Fig. 6.
5 6
TOTAL
ATTENUATION
/
/
CONDUCTANCE
LOSS
400 600 800 1000 1200 1400
FREQUENCY IN KILOCYCLES PER SECOND
1600
Fig. 6 — Attenuation of small flexible coaxial structure (Fig. 2).
The figure shows also that the conductance loss due to the insulation
is a small part of the total.
It is interesting to compare the curves of the transmission character-
istics of the coaxial circuit with those of other types of circuits. Figure
WIDE-BAND TRANSMISSION OVER COAXIAL LINES 663
.?. 5
NO. 19 GAUGE
CABLE PAIR
'NO. 16 GAUGE
CABLE PAIR
/
/
/
/
/
0.3 INCH
COAXIAL .^
/
/
/
/
OPEN
WIRE
2.5
COA
NCH
<IAL \
1 ♦
J
200
300 400 500 600 700
FREQUENCY IN KILOCYCLES PER SECOND
Fig. 7 — Attenuation frequency characteristics of coaxial and other circuits.
7 shows the high-frequency attenuation of two sizes of coaxial circuit
using copper tube outer conductors, of .3 inch and 2.5 inch inner diame-
ter, and that of cable and open-wire pairs in the same frequency range.
Effect of Eccentricity
The small effect of lack of perfect coaxiality upon the attenuation
of a coaxial circuit is illustrated by the curve of Fig. 8, which shows
664
BELL SYSTEM TECHNICAL JOURNAL
TAGE INCREASE IN ATTENUATION
OVER COAXIAL CASE
PERCEN
.0
2 .04 .06 .08 .10 12 14 .16 .18 .2C
RATIO OF DISTANCE BETWEEN CONDUCTOR AXES
TO INNER RADIUS OF OUTER CONDUCTOR
Fig. 8 — Increase in attenuation of coaxial circuit due to eccentricity.
attenuation ratios plotted as a function of eccentricity, assuming a
fixed ratio of conductor diameters and substantially air insulation.
Temperature Coefficient
With a coaxial circuit, as with other types of circuits, the tempera-
ture coefficient of resistance decreases as the frequency is increased,
due to the action of skin effect, and approaches a value of one-half the
d.-c. temperature coefficient. 12 Thus, for conductors of copper the
a.-c. coefficient at high frequencies is approximately .002 per degree
Centigrade. When the dielectric losses are small, the temperature
coefficient of attenuation at high frequencies is the same as the tempera-
ture coefficient of resistance.
Diameter Ratio
An interesting condition exists with regard to the relative sizes of
the two conductors. For a given size of outer conductor there is a
unique ratio of inner diameter of outer conductor to outer diameter of
inner conductor which gives a minimum attenuation. At high fre-
quencies, this optimum ratio of diameters (or radii) is practically inde-
pendent of frequency. When the conductivity is the same for both
conductors, and either the dielectric losses are small or the insulation
is distributed so that the dielectric flux follows radial lines, the value
of the optimum diameter ratio is approximately 3.6. When the outer
and inner conductors do not have the same conductivity, the optimum
diameter ratio differs from this value. For a lead outer conductor and
copper inner conductor, for example, the ratio should be about 5.3.
WIDE-BAND TRANSMISSION OVER COAXIAL LINES 665
Stranding
Inasmuch as the resistance of the inner conductor contributes a
large part of the high frequency attenuation of a coaxial circuit, it is
natural to consider the possibility of reducing this resistance by employ-
ing a conductor composed of insulated strands suitably twisted or
interwoven. 13 Experiments along this line showed that this method
is impractical at frequencies above about 500 kilocycles, owing to the
fineness of stranding required.
Characteristic Impedance
The high-frequency characteristic impedance of a coaxial circuit
varies inversely with the square root of the effective dielectric constant,
i.e., the ratio of the actual capacitance to the capacitance that would
be obtained with air insulation. The impedance of a circuit having a
given dielectric constant depends merely upon the ratio of conductor
diameters and not upon the absolute dimensions. For a diameter
ratio of 3.6, the impedance of a coaxial circuit with gaseous insulation
is about 75 ohms.
Velocity of Propagation
For a coaxial circuit with substantially gaseous insulation, the veloc-
ity of propagation at high frequencies approaches the speed of light.
Hence the circuit is capable of providing high velocity telephone chan-
nels with their well-recognized advantages. The fact that the ve-
locity at high frequencies is substantially constant minimizes the
correction required to bring the delay distortion within the limits
required for a high quality television band.
Shielding and Crosstalk
The shielding effect of the outer conductor of a coaxial circuit is
illustrated in Fig. 9, where the transfer impedance between the outer
and inner surfaces of the outer conductor is plotted as a function of
frequency. There will be observed the sharp decrease in inductive
susceptibility as the frequency rises. On this account, the crosstalk
between adjacent coaxial circuits falls off very rapidly with increasing
frequency. The trend is, therefore, markedly different from that for
ordinary non-shielded circuits which rely upon balance to limit the
inductive coupling. As a practical matter, less shielding is ordinarily
required to avoid crosstalk than to avoid external interference.
With suitable design the shielding effect of the outer conductor
renders the coaxial circuit substantially immune to external inter-
ference at frequencies above the lower end of the spectrum. Hence
the signals transmitted over the circuit may be permitted to drop
666
BELL SYSTEM TECHNICAL JOURNAL
o
o
2
aO 40
Q- Z
.o
b.„ 50
60
5£ 70
'u 80
'£100
no
0.3"DIAM. 30- MIL
^ COPPER WALL
2.5"DIAM. 60-MIL
COPPER WALL
s
s
\
\
\
\
\
\
\
\
20 50 100 200 500 1000 2000 5000
FREQUENCY IN KILOCYCLES PER SECOND
Fig. 9 — Transfer impedance of coaxial circuit.
down to a level determined largely by the noise due to thermal agita-
tion of electricity in the conductors and tube noise in the associated
amplifiers. It appears uneconomical to make the outer conductor
sufficiently thick to provide adequate shielding for the very low fre-
quencies. Also it seems impractical to design the repeaters to trans-
mit very low frequencies. Hence the best system design appears to
be one in which the lowest five or ten per cent of the frequency range
is not used for signal transmission. The coaxial circuit is, however,
well suited to the transmission of 60-cycle current for operating re-
peaters, a matter which will be referred to later.
Broad-Band Amplifiers
In order to realize the full advantage of broad-band transmission,
the repeater for this type of system should be capable of amplifying
the entire frequency band en bloc. Furthermore, it should be so stable
and free from distortion that a large number of repeaters may be op-
erated in tandem. Although high-gain radio frequency amplifiers are
in everyday use, these are generally arranged to amplify at any one
time only a relatively narrow band of frequencies, a variable tuning
device being provided so that the amplification may be obtained at
WIDE-BAND TRANSMISSION OVER COAXIAL LINES 667
any point in a fairly wide frequency range. The high gain is usually
obtained by presenting a high impedance to the input circuits of the
various tubes through tuning the input and interstage coupling cir-
cuits to approximate anti-resonance.
In amplifying a broad band of frequencies, it is difficult to maintain
a very high impedance facing the grid circuits. The inherent capaci-
tances between the tube elements and in the mounting result in a
rather low impedance shunt which can not be resonated over the de-
sired frequency band. It is, therefore, necessary to use relatively low
impedance coupling circuits and to obtain as high gain as possible
from the tubes themselves. The amount of gain which can be ob-
tained without regeneration depends, of course, upon the type of tube,
the number of amplification stages, the band width, and also upon the
ratio of highest to lowest frequency transmitted.
Repeater Gain
The total net gain desired in a line amplifier is such as to raise the
level of an incoming signal from its minimum permissible value, which
is limited by interference, up to the maximum value which the ampli-
fier can handle.
As pointed out above, the noise in a well shielded system is that due
to resistance noise in the line conductors and tube noise in the ampli-
fiers. In some of the repeaters which have been built, the amplifier
noise has been kept down to about 2 db above resistance noise, corre-
sponding to about 7 X 10~ 17 watt per voice channel. In a long line
with many repeaters the noise voltages add at random, or in other
words, the noise powers add directly. Assuming, for example, a line
with 200 repeaters, the noise power at the far end would be 200 times
that for a single repeater section. In general, the line and amplifier
noise will not be objectionable in a long telephone channel if the speech
sideband level at any amplifier input is not permitted to drop more
than about 55 db below the level of the voice frequency band at the
transmitting toll switchboard.
The determination of the volume which a tube can handle in trans-
mitting a wide band of frequencies involves a knowledge of the distri-
bution in time and frequency of the signaling energy and of the require-
ments as to distortion of the various components of the signal. The
distribution of the energy in telephone signals has been the subject of
much study. This distribution is known to vary over very wide limits,
depending upon the voice of the talker and many other factors. It is,
therefore, obvious that the problem of summing up the energy of some
hundreds of simultaneous telephone conversations is a difficult one.
668
. BELL SYSTEM TECHNICAL JOURNAL
Enough work has been done, however, to indicate fairly well what the
result of such addition will be.
As to distortion in telephone transmission, the most serious problem
has been to limit the intermodulation between various signals which
are transmitted simultaneously through the repeater and appear as
noise in the telephone channel. The requirement for such noise is
similar to that for line and tube noise, and similarly it will add up in
successive repeater sections for a long line. With present types of
tubes operating with a moderate plate potential, the modulation re-
quirement can be met only at relatively low output levels. To im-
prove this situation and also to obtain advantages in amplifier stabil-
ity, the reversed feedback principle employed for cable carrier ampli-
fiers, as described in a paper by H. S. Black, 14 has been extended to
higher frequency ranges. It has been found that amplifiers of this
type having 30 db feedback reduce the distortion to such an extent
that each amplifier of a long system carrying several hundred telephone
channels will handle satisfactorily a channel output signal level about
5 db above that at the input of the toll line.
The maximum gain which can be used in the repeater, therefore, is,
in the illustrative case given above of a long system carrying several
hundred telephone channels, the difference between the minimum and
maximum levels of 55 db below and 5 db above the point of reference,
respectively, or a total gain of 60 db. (With a .3-inch coaxial line of
the type shown in Fig. 2, this corresponds to a repeater spacing of
about 10 miles.) If a repeater is to have 60 db net gain and at the
PRE- INPUT
EQUALIZER TRANSFORMER
INTERSTAGE
/COUPLINGS^
OUTPUT
TRANSFORMER
@^2
TO POWER
EQUIPMENT
Fig. 10 — Circuit of 1000-kilocycle three-stage feedback repeater.
WIDE-BAND TRANSMISSION OVER COAXIAL LINES
669
same time about 30 db feedback, it is obvious that the total forward
gain through the amplifying stages must be about 90 db. The circuit
of an experimental amplifier meeting the gain requirements for a
frequency band from 50 to 1000 kilocycles is shown schematically in
Fig. 10.
Gain- Frequency Characteristic
As pointed out above, the line attenuation is not- uniform with fre-
quency. For a repeater section which has a loss of, say, 60 db at
1000 kilocycles, the loss at 50 kilocycles would be only about 15 db.
Such a sloping characteristic can be taken care of either by designing
the repeater to have an equivalent slope in its gain-frequency charac-
teristic or by designing it for constant gain and supplementing it with
an equalizer which gives the desired overall characteristic. Both
methods have been tried out, as well as intermediate ones. Figure 11
LINE —DESIRED CHARACTERISTIC
POINTS — ACTUAL CHARACTERISTIC
NO TEMPERATURE REGULATION
100 200 300 400 500 600 700 800 900 1000 1100
FREQUENCY IN KILOCYCLES PER SECOND
Fig. 11 — Gain of 1000-kilocycle repeater compared with line characteristic.
illustrates such a sloping characteristic obtained by adjusting the
coupling impedances in a three-tube repeater, designed in this case for
60 db gain at 1000 kilocycles. The accompanying photograph, Fig.
12, gives an idea of the apparatus required in such a repeater, apart
from the power supply equipment.
Regulation for Temperature Changes
It is necessary that the repeater provide compensation for varia-
tions in the line attenuation due to changes of temperature. In the
case of aerial construction such variations might amount to as much
as 8 per cent in a day or 16 per cent in a year. If the line is under-
670
BELL SYSTEM TECHNICAL JOURNAL
ground the annual variation is only about one-third of the above
value and the changes occur much more slowly. On a transcontinental
line the annual variation might total about 1500 db. Inasmuch as it
is desirable to hold the transmission on a long circuit constant within
about ± 2 db, it is obvious that the regulation problem is a serious one.
In a single repeater section of aerial line the variation might amount
to ± 2.5 db per day or ± 5 db per year. Such variations, if allowed
Fig. 12 — Photograph of 1000-kilocycIe repeater.
to accumulate over several repeater sections, will drop the signal down
into the noise or raise it so as to overload the tubes. It is, therefore,
advisable to provide some regulation at every repeater in an aerial
line so as to maintain the transmission levels at approximately their
correct position. For underground installations the regulating mech-
anism may be omitted on two out of every three repeaters.
In choosing a type of regulator system the necessity for avoiding
cumulative errors in the large number of repeater sections has been
borne in mind. In view of the wide band available, a pilot channel
regulator system was naturally suggested. Such a scheme employing
two pilot frequencies has been used experimentally to adjust the gain
characteristic in such a way as to maintain the desired levels through-
out the band. The accuracy with which this has been accomplished
for a single repeater section is illustrated in Fig. 13. Over the entire
band of frequencies and the extreme ranges in temperature which may
be encountered, the desired regulation is obtained within a few tenths
of a db.
WIDE-BAND TRANSMISSION OVER COAXIAL LINES
671
50
<-> 40
10
j?^"
^
^
o*
LINES- DESIRED CHARACTERISTICS
POINTS -ACTUAL CHARACTERISTICS
,
tp
100
300 400 500 €O0 700 800 900
FREQUENCY IN KILOCYCLES PER SECONf-
1000 1100
Fig. 13 — Temperature regulation — line and repeater characteristics.
Repeater Operation, Power Supply, Housing, Etc.
In view of the large number of repeaters required in a broad-band
transmission system it is essential that the repeater stations be simple
.and involve a minimum of maintenance. With the repeater design
as described it is expected that most of the repeaters may be operated
on an unattended basis, requiring maintenance visits at infrequent
intervals.
An important factor in this connection is the possibility of supplying
current to unattended repeaters over the transmission line itself. The
coaxial line is well adapted to transmit 60-cycle current to re-
peaters without extreme losses and without hazard. The repeaters
with regulating arrangements as built experimentally for a million-
cycle system are designed to use 60-cycle current, which in this case
appears to have the usual advantages over d.-c. supply. One repeater
requires a supply of about 150 watts. The number of repeaters which
can be supplied with current transmitted over the line from any one
point depends upon the voltage limitation which may be imposed on
the circuit from considerations of safety.
For a repeater of the type described with current supplied over the
line, only a very modest housing arrangement will be required. For
the great majority of stations, it appears possible to accommodate the
repeaters in weatherproof containers mounted on poles, in small huts,
or in manholes.
^
672
BELL SYSTEM TECHNICAL JOURNAL
Higher Frequency Repeaters
Most of what has been said above applies particularly to repeaters
transmitting frequencies up to about 1000 kilocycles. However,
study has been given also to repeaters, both of the feedback and the
non-feedback type, for transmitting higher frequencies. Experimental
repeaters covering the range from 500 to 5000 kilocycles have been built
and tested. These were capable of handling simultaneously the full
complement of over 1000 channels which such a broad band will
permit. The frequency characteristic of one of these repeaters, and the
measured attenuation of a section of line of the type tested at Phoenix-
ville are shown in Fig. 14.
H45
a.
O
Z 35
<
30
*
ff
MEASURED ATTENUATION
OF 2>*>" CONDUCTOR .
(PHOENIXVILLE TYPE) >^
s /
S
/j
// X
V AMPLIFIER
' GAIN
jr
//
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
FREQUENCY IN MEGACYCLES PER SECOND
Fig. 14 — Frequency characteristic of coaxial line and 5000-kilocycle repeater.
Terminal Arrangements
In order to utilize a broad band effectively, for telephone purposes,
the speech channels must be placed as close together in frequency as
practicable. The factors which limit this spacing are: (1) The width of
WIDE- BAND TRANSMISSION OVER COAXIAL LINES
673
speech band to be transmitted and (2), the sharpness of available
selecting networks.
As to the width of speech band, the present requirement for commer-
cial telephone circuits is an effective transmission band width of at
least 2500 cycles, extending from 250 to 2750 cycles. It has been found
that a band of this width or more may be obtained with channels
spaced at 4000-cycle intervals. Band filters using ordinary electrical
elements are available, 3 for selecting such channels in the range from
zero to about 50 kilocycles. Channel selecting filters using quartz
crystal elements Ui 1G have been developed in the range from about 30
to 500 kilocycles. The selectivity of a typical filter employing quartz
crystal elements is shown on Fig. 15.
9
70
£ 30
20
10
1
V
^
u
/
1 DECIE
a. A
*- 2850 CYCLE BAND-H/
70 71 72 73 74 75 76 77 78 79
FREQUENCY IN KILOCYCLES PER SECOND
Fig. 15 — Frequency characteristic of quartz crystal channel hand filter.
Initial Step of Modulation
The initial modulation (from the voice range) may be carried out in
an ordinary vacuum tube modulator or one of a number of other non-
linear devices. The method chosen for the present experimental work
674
BELL SYSTEM TECHNICAL JOURNAL
employs a single sideband with suppressed carrier, using a copper-oxide
modulator associated with a quartz crystal channel filter. The
terminal apparatus required for two-way transmission over a two-
path circuit is shown diagrammatically on the left-hand side of Fig. 16.
OTHER
TRANSMITTING
CHANNEL
BAND FILTERS
(64-108 KC)
OF THIS GROUP
CHANNEL
MODULATOR
t
168-72 l-f
! KC j~r
|64-68 ri
! KC !--»-
TELEPHONE
SET '
HYBRID
000000
-^Tj
O^— NET -
SSu —
COIL
V
WORK
60-64
KC
GROUP
MODULATOR
fir— i
H 876- !
. H 924- !
l L J
I TRANS-
I MITTING
'AMPLIFIER
60-108
KC
TRANSMITTING
CHANNEL
BAND FILTER
DEMODULATOR
AMPLIFIER
64 KC
972-
1020
KC
1080
KC
RECEIVING
CHANNEL
BAND FILTER
60-64
KC
CHANNEL
DEMODULATOR
OTHER
RECEIVING
CHANNEL
BAND FILTERS S r -,
(64-108 KC) j 68-72 |--f
OF THIS GROUP 1 1 KC fT
i 64-68 j-t
■ KC h+
TRANSMITTING
GROUP
BAND FILTER
RECEIVING
GROUP
BAND FILTER
OTHER
TRANSMITTING
GROUP
BAND FILTERS
EAST
{— V Qiinm, h
COAXIAL LINE
60-1020 KC
COAXIAL LINE
60-1020 KC
60-108
KC
972-
1020
KC
GROUP
DEMODULATOR
rtF
WEST
i RECEIVING
' AMPLIFIER
. f-! 972- !
T--T-J924KCJ
. H 92 "- 1
r4- J ,876KC!
t Y
OTHER
RECEIVING
GROUP
BAND FILTERS
Fig. 16— -Schematic of four-wire circuit employing two steps of modulation.
A frequency allocation which has been used for experimental pur-
poses employs carriers from 64 to 108 kilocycles for the initial step of
modulation. The lower sidebands are selected and placed side by
side in the range from 60 to 108 kilocycles, as illustrated in Fig. 17,
forming a group of 12 channels.
Double Modulation
In order to extend the frequency range of a system to accommodate
a very large number of channels, it appears to be more economical to
add a second step of modulation rather than carry the individual
channel modulation up to higher frequencies. Such a second step of
modulation has been used experimentally to translate the initial group
of 12 channels en bloc from the range 60 to 108 kilocycles up to higher
frequencies. It is possible to place such groups of channels one above
another as illustrated in the upper part of the diagram of Fig. 18, up
WIDE- BAND TRANSMISSION OVER COAXIAL LINES 675
VOICE
FREQUENCY
CHANNELS
10
W If " 'I f " ' ' '
4
FREQUENCY IN I KILOCYCLES PER SECOND
U 48 KC GROUP -
Fig. 17 — Diagram illustrating frequency allocation for first step of modulation.
; i i i
780 J 1020 4 1260 4 1500
FREQUENCY IN KILOCYCLES PER SECOND
I 240 KC I
GROUPS
Fig. 18 — Diagram illustrating frequency allocation for two or three steps of
modulation.
676 BELL SYSTEM TECHNICAL JOURNAL
to about 1000 kilocycles, wasting no frequency space between groups
and thus keeping the channels spaced at intervals of 4 kilocycles
throughout the entire range.
The apparatus required for this purpose is shown schematically in
Fig. 16, which illustrates the complete terminal arrangements for a
single channel employing double modulation. The figure indicates by
dottled lines where the other channels and groups of channels are con-
nected to the system.
A modulator for shifting the frequency position of a group of chan-
nels inherently yields many different modulation products as a result
of the intermodulation of the signal frequencies with the carrier fre-
quency and /or with one another. Out of these products only the
"group sideband " is desired. The number of the modulation products
resulting merely from the lower ordered terms of the modulator re-
sponse characteristic is extremely large. All such products must be
considered from the standpoint of interference either with the group
which is wanted in the output or with other groups to be transmitted
over the system. Various expedients may be used to avoid inter-
ference as follows: (1) A proper choice of frequency allocation will
place the undesired modulation products in the least objectionable
location with respect to the wanted signal bands; (2) a high ratio of
carrier to signal will minimize all products involving only the signal
frequencies; (3) the use of a balanced modulator will materially reduce
all products involving the second order of the signal ; (4) selectivity in
the group filters will tend to eliminate all products removed some dis-
tance from the wanted signal group. Giving due regard to these
factors, balanced vacuum tube group modulators have been developed
which are satisfactory for the frequency allocations employed.
Triple Modulation
For systems involving frequencies higher than about 1000 kilo-
cycles it may be desirable to introduce a third step of modulation.
In some experiments along this line a "super-group" of 60 channels,
or five 12-channel groups, has been chosen. The lower part of Fig.
18 illustrates, for a triple modulation system, the shifting of super-
groups of 60 channels each to the line frequency position. This
method has been employed experimentally up to about 5,000 kilo-
cycles. It is of interest to note that even in extending these systems
to such high frequencies, channels are placed side by side at intervals
of 4000 cycles to form a practically continuous useful band for trans-
mission over the line.
WIDE-BAND TRANSMISSION OVER COAXIAL LINES 677
Demodulation
On the receiving side the modulation process is reversed. The
apparatus units are similar to those used on the transmitting side, and
are similarly arranged. Figure 16 illustrates this for the case of double
modulation.
Carrier Frequency Supply
In systems operating at higher frequencies it is necessary that the
carrier frequencies be maintained within a few cycles of their theoretical
position in order to avoid beat tones or distortion of the speech band.
Separate oscillators of high stability could, of course, be used for
the carrier supply but it appears more economical to provide carriers
by means of harmonic generation from a fundamental basic frequency.
Such a base frequency may be transmitted from one end of the cir-
cuit to the other, or may be supplied separately at each end.
Television
The broad band made available by the line and repeaters may be
used for the transmission of signals for high-quality television. Such
signals may contain frequency components extending over the entire
range from zero or a very low frequency up to a million or more cycles. 4
The amplifying and transmitting of these frequencies, particularly the
lower ones, presents a serious problem. The difficulty can be over-
come by translating the entire band upward in frequency to a range
which can be satisfactorily transmitted. To effect such a shift, the
television band may first be modulated up to a position considerably
higher than its highest frequency and then with a second step of modu-
lation be stepped down to the position desired for line transmission.
This method is illustrated in Fig. 19 for a 500-kc. television signal
band. The original television signal is first modulated with a rela-
tively high frequency, two million cycles in this case (Ci). The lower
sideband, extending to 1500 kilocycles, is selected and is modulated
again with a frequency of 2100 kilocycles (C 2 ). The lower sideband
of 1 00 to 600 kilocycles is selected with a special filter so designed that
the low frequency end is accurately reproduced. The television
signal then occupies the frequency range of 100 to 600 kilocycles as
shown on the diagram and may be transmitted over a coaxial or other
high frequency line. At the receiving end a reverse process is em-
ployed. The same method using correspondingly higher frequencies
may be used for wider bands of television signals.
678
BELL SYSTEM TECHNICAL JOURNAL
FREQUENCY IN KILOCYCLES PER SECOND
2000 KC
CARRIER
2100 KC
CARRIER
PHOTO- rfT-
ELECTRIC ( )
CELL ^£L_
,ST
FILTER
,ST
MODULATOR
FILTER
2ND
MODULATOR
EQUALIZER
Fig. 19 — Double modulation method for translating television signals for wire line
transmission.
Other Communication Facilities
The telephone channels provided by the system may be used for
other types of communication services, such as multi-channel tele-
graph, teletype, picture transmission, etc. For the transmission of a
high-quality musical program, which requires a wider band than does
commercial telephony, two or more adjacent telephone channels may
be merged. The adaptability of the broad-band system to different
types of transmission thus will be evident.
As already noted, the commercial application of these systems for
wide-band transmission over coaxial lines must await a demand for
large groups of communication facilities or for television. The re-
sults which have been outlined are based upon development work in
the laboratory and the field, and it is probable that the systems when
used commercially will differ considerably from the arrangements
described.
WIDE-BAND TRANSMISSION OVER COAXIAL LINES 679
References
1. E. H. Colpitts and 0. B. Blackwell, "Carrier Current Telephone and Teleg-
raphy," A. I. E. E. Trans., Vol. 40, February 1921, p. 205-300.
2. H. A. Affel, C. S. Demarest, and C. W. Green, "Carrier Systems on Long Dis-
tance Telephone Lines," A. I. E. E. Trans., Vol. 47, October 1928, 1360-1367.
Bell Sys. Tech. Jour., Vol. VII, July 1928, p. 564-629.
3. A. B. Clark and B. W. Kendall, "Communication by Carrier in Cable," Elec.
Engg., Vol. 52, July 1933, p. 477-481, Bell Sys. Tech. Jour., Vol. XII, July 1933,
p. 251-263.
4.' P. Mertz and F. Gray, "Theory of Scanning and Its Relation to the Characteris-
tics of the Transmitted Signal in Telephotography and Television," Bell Sys.
Tech. Jour., Vol. XIII, July 1934, p. 464.
5. E. W. Engstrom, "A Study of Television Image Characteristics," Proc. I. R. E.,
Vol. 21, December 1933, p. 1631-1651.
6. S. A. Schelkunoff, "The Electromagnetic Theory of Coaxial Transmission Lines
and Cylindrical Shields." Bell Sys. Tech. Jour., Vol. XIII, October 1934.
7. J. R. Carson and J. J. Gilbert, "Transmission Characteristics of the Submarine
Cable," Jour. Franklin Institute, Vol. 192, December 1921, p. 705-735.
8. W. H. Martin, G. A. Anderegg and B. W. Kendall, "The Key West-Havana
Submarine Telephone Cable System," Trans. A. I.E.E., Vol.41, 1922, p. 1-19.
9. British Patent No. 284,005, C. S. Franklin, January 17, 1928.
10. E. J. Sterba and C. B. Feldman, "Transmission Lines for Short-Wave Radio
Systems," J. R. E. Proc, Vol. 20, July 1932, p. 1163-1202; also Bell Sys. Tech.
Jour., Vol. II, July 1932, p. 411-450.
11. U. S. Patents No. 1,835,031, L. Espenschied and H. A. Affel, December 9, 1931,
and No. 1,941,116, M. E. Strieby, December 26, 1933.
12. E. I. Green, "Transmission Characteristics of Open-Wire Lines at Carrier
Frequencies," A. I. E. E. Trans., Vol. 49, October 1930, p. 1524-1535; Bell
Sys. Tech. Jour., Vol. IX, October 1930, p. 730-759.
13. H. A. Affel and E. I. Green, U. S. Patent No. 1,818,027, Aug. 11, 1931.
14. H. S. Black, "Stabilized Feed-Back Amplifiers," Electrical Engineering, Vol. 53,
January 1934, p. 114-120. Bell Sys. Tech. Jour., Vol. XIII, January 1934,
p. 1-18.
15. L. Espenschield, U. S. Patent No. 1,795,204, March 3, 1931.
16. W. P. Mason, "Electrical Wave Filters Employing Quartz Crystals as Elements,"
Bell Sys. Tech. Jour., Vol. XIII, July 1934, p. 405.
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